In recent years, cut down of the amount of carbon dioxide has been ardently desired in order to cope with air pollution and global warming. In the motor vehicle industry, cut down of carbon dioxide emissions due to introduction of electric vehicles (EV) and hybrid electric vehicles (HEV) has been highly expected, and development of electric devices such as secondary batteries for driving motors, which holds the key to practical use thereof has been actively carried out.
The secondary batteries for driving motors are required to exhibit extremely high-output characteristics and high energy as compared to consumer lithium ion secondary batteries to be used in mobile phones, notebook computers, and the like. Hence, lithium ion secondary batteries having the highest theoretical energy among all the batteries have attracted attention, and development thereof has been rapidly advanced at present.
A lithium ion secondary battery generally has a configuration in which a positive electrode having a positive electrode active material and the like coated on both sides of a positive electrode current collector by using a binder and a negative electrode having a negative electrode active material and the like coated on both sides of a negative electrode current collector by using a binder are connected to each other via an electrolyte layer to be housed in a battery case.
Hitherto, a carbon and graphite-based material, which is advantageous from the viewpoint of lifespan of charge and discharge cycles and cost, has been used in the negative electrode of a lithium ion secondary battery. However, in the case of a carbon and graphite-based negative electrode material, since charge and discharge proceed by occlusion and release of lithium ions into and from the graphite crystals, there is a disadvantage that a charge and discharge capacity that is equal to or higher than the theoretical capacity, 372 mAh/g, to be obtained from LiC6, the maximum lithium-introduced compound, cannot be obtained. For this reason, it is difficult to obtain a capacity and an energy density which satisfy the practical use level of a vehicle application by using a carbon and graphite-based negative electrode material.
In contrast, a battery using a material to be alloyed with Li in the negative electrode is expected as a negative electrode material in a vehicle application since the energy density is improved as compared to a conventional carbon and graphite-based negative electrode material. For example, an Si material occludes and releases 3.75 mol of lithium ions per 1 mol as in the following Reaction Formula (A) in charge and discharge, and the theoretical capacity is 3600 mAh/g in Li15Si4 (═Li3.75Si).[Chemical Formula 1]Si+3.75Li++e−⇄Li3.75S  (A)
However, in a lithium ion secondary battery using a material to be alloyed with Li in a negative electrode, expansion and contraction of the negative electrode during the charge and discharge is great. For example, a volume expansion in the case of occluding a Li ion is about 1.2 times for a graphite material, while a great volume change (about 4 times) occurs for a Si material since the amorphous state is converted to a crystalline state when Si and Li are alloyed, and there is thus a problem that the cycle lifespan of the electrode decreases. In addition, in the case of an Si negative electrode active material, the capacity and the cycle durability have a trade-off relationship, and there is thus a problem that it is difficult to improve the cycle durability while having a high capacity.
Here, WO 2006/129415 A discloses an invention aimed to provide a nonaqueous electrolyte secondary battery including a negative electrode pellet having a high capacity and an excellent cycle lifespan. Specifically, it is disclosed that a silicon-containing alloy which is obtained by mixing a silicon powder and a titanium powder by a mechanical alloying method and wet pulverizing the mixture, and has a first phase containing silicon as a main body and a second phase containing a silicide of titanium (TiSi2 or the like) is used as a negative electrode active material. It is also disclosed that at least either of these two phases is amorphous or low crystalline.